All Linux clients offline

urBackup server: 2.4.15 and 2.5.26
OS: Ubuntu 18.04.6
urBackup clients: 2.4.10.0
OS: Ubuntu 22.04.1 and 18.04.6

All Linux clients stopped to be online on the same day. So the server seemed to be the problem, but only security updates run. And the Windows clients still work flawless. So i tried to reconnect by deleting the server_idents.txt despite the key inside corresponding to the server and restart the client. No server_idents.txt was created and the client stayed offline. Nothing in the client logs with debug on. I also checked the ufw ports and even disabled it temporarly on both sides. Nothing.
So I took the chance to upgrade both sides. Didn’t help. Except the server_idents.txt was created but with no key inside.
Is there still something I can do beside removing the client on the server? Do I loose the backup, when I do?

Manually removing and adding the client does also not work. The interesting part is that bellow Client discovery hints the client is shown as online for a short time. But doesn’t get added. After that it is shown offline again in Client discovery hints. Never added.
Reinstalling did also not help.
It is still possible to add new Linux clients.
Server/Linux client filesystem: BTRFS

The server/client logs in debug mode show nothing special. During a restart it shows that it discovers the windows clients and the freshly added Linux clients, but not the older Linux clients.

At the day where all Linux clients stopped communication with the urbackup server the urbackup server seemed crashed. After a server restart the windows clients reconnected, Linux not.

It seems that the removed clients are still in the database.

Would appreciate it if somebody has an idea or a tested SQL statement to remove the stale clients and there associated data from the database.